1. Field of the Invention
This invention relates to an improved servo-motor control system and, more particularly, to an improved system for controlling a stepping motor in order to conserve energy when the motor is in its equilibrium position.
2. Description of the Prior Art
In the field of analog servo systems, heretofore, a d.c. servo-motor of the so-called stepping motor type has been driven in response to a signal which corresponds to an analog differential input. Such systems have obtained good reputations because of their simplified construction and high reliability, particularly when used in combination with contactless d.c. motors. Such systems are also widely used, in part, because of the rotor of the motor which can hold its position at a predetermined angular position.
FIG. 1 is a block diagram showing a typical prior art circuit for driving a servo-motor. In the figure, reference numeral 1 designates a stepping servo-motor capable of bi-directional rotation, numeral 2 a differential amplifier, numeral 3 an input terminal to which an analog signal is applied, numeral 4 an oscillator which transmits one or another of two pulse trains upon the receipt of the output of said differential amplifier 2, and numeral 5 represents an energization circuit which includes a plurality of bi-stable circuits. The rotational direction of the motor is determined by which one of the two pulse trains is transmitted from oscillator 4.
The rotor of servo-motor 1 rotates through a predetermined angle upon receipt of each pulse of a pulse train for forward or backward rotation from the pulse transmitter 4 and stops rotating when pulse signals are stopped. The rotor is then held in the balanced position which is established last owing to the magnetic forces between the motor rotor and stator.
However, the stepping servo-motor in such prior art constructions are always connected to the power source regardless of whether it is stepping, or in a balanced condition; thus, the power consumed is considerable and the temperature of the motor itself can become excessively elevated as well as parts other than the motor, for instance, the electronic circuits.
Generally, it is rare for such a servo-motor to step continuously. Typically, such motors remain in a balance position for long intervals, particularly in applications of such servo-motors to analog instruments or the like. In such application, however, a load for the servo-motor, for example a slide type variable resistor, often provides a holding function which tends to maintain the motor in its balanced position. In addition, the permanent magnet in the rotor of the servo-motor also provides a holding force to some extent. Consequently, the balance position of the motor in such applications can be maintained even if the power supply for energizing the motor is cutoff.
In the prior art, accordingly, the points mentioned above have been considered and utilized; that is, the prior art has proposed a method in which the servo-motor is energized for a predetermined period in accordance with trigger pulses while the power is always in a cutoff condition for periods other than the above predetermined period. In addition, there has been a proposal for another method in which a comparator has an insensitive or dead zone which functions to cutoff the power for energizing the servo-motor through the operation of a switch circuit when an input error signal is below a predetermined level. However, these prior art proposals encounter certain difficulties in their implementation; for example, specific one-shot type multi-vibrator circuits or the comparator having insensitive zone is required.